We have some security systems which prevent access from most bots. I can assure you that the crawlers of all popular search engines are not affected by this, but I don’t think most SEO checkers work so well. So I suspect the SEO checkers are not looking at your website, but at the interstitial security page.
Actually, to quote Nibbler’s own About page:
Some websites use special anti-bot technologies to stop spammers. These websites may also stop Nibbler from being able to test them.
I think that’s what’s happening here as well.
Just use Google’s own SEO checker, rather than some external tool which pretends to know exactly how Google ranks a site.